In 1776, the year the American Revolution began, the known solar system was a tidy place. Six planets: Mercury, Venus, Earth, Mars, Jupiter, and Saturn. All visible to the naked eye, all known to ...

What 250 years has done to our solar system planet count. When you purchase through links on our site, we may earn an affiliate commission. Here’s how it works.

For millions of years, a frozen wanderer drifted between the stars before slipping into our solar system as 3I/ATLAS—only the third known interstellar comet ever spotted. When scientists turned NASA’s ...